X rays are a powerful probe for investigating metal and radionuclide transformations in soils, sediments, and groundwaters. In particular, synchrotron-based X-ray investigations can identify the changes in an element’s valence state and chemical speciation that often result from microbially mediated electron transfer. This chapter describes some of the synchrotron-based X-ray techniques (X-ray absorption spectroscopy, X-ray fluorescence, and X-ray microscopy) that can be used to improve understanding of metal transformations. The X-ray absorption near-edge structure (XANES) technique provides an in situ probe of an element’s oxidation state and clearly can contribute significantly to an understanding of the fate of elements in the environment, in both solid and solution phases. In addition to its utility, XANES is relatively easy to implement and has been perhaps the most commonly used synchrotron-based X-ray technique for monitoring metal transformations in environmental studies. XANES spectroscopy focuses on the energy range near an element's absorption edge, which is related to the element's valence state. Extended X-ray absorption fine-structure (EXAFS) focuses on the energy region well above the absorption edge and yields information on the local chemical environment of the absorbing element. Investigators can initiate the use of synchrotron radiation in their research in a number of ways.

(A) Incident X-ray intensity as a function of X-ray energy. (B) Transmitted X-ray intensity as a function of X-ray energy, showing the drop in transmission due to an increase in X-ray absorption by U at the LIII absorption edge (17,166 eV). (C) U LIII edge X-ray absorption coefficient, obtained as the natural log ratio of the incident (A) to transmitted (B) X-ray intensities. (D) Normalized X-ray absorption U LIII edge data for a U(IV) standard (open symbols), a U(VI) standard (open triangles), a U(VI) sample without ethanol for bioreduction (U–ETOH, thick gray line), and a U(VI) sample with ethanol for bioreduction (U+ETOH, thin gray line). The energy value of the absorption edge (between the arrows) is related to the average valence state of U. The U+ETOH sample has a major U(IV) component, while the U-ETOH sample is mostly U(VI).

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FIGURE 2

(A) Incident X-ray intensity as a function of X-ray energy. (B) Transmitted X-ray intensity as a function of X-ray energy, showing the drop in transmission due to an increase in X-ray absorption by U at the LIII absorption edge (17,166 eV). (C) U LIII edge X-ray absorption coefficient, obtained as the natural log ratio of the incident (A) to transmitted (B) X-ray intensities. (D) Normalized X-ray absorption U LIII edge data for a U(IV) standard (open symbols), a U(VI) standard (open triangles), a U(VI) sample without ethanol for bioreduction (U–ETOH, thick gray line), and a U(VI) sample with ethanol for bioreduction (U+ETOH, thin gray line). The energy value of the absorption edge (between the arrows) is related to the average valence state of U. The U+ETOH sample has a major U(IV) component, while the U-ETOH sample is mostly U(VI).

U LIII edge EXAFS data from an aqueous uranyl carbonate species. (A) X-ray absorption data (symbol) and background function (line). (B) EXAFS data, x(k) · k, obtained as the difference between the X-ray absorption data and the background function shown in panel A, with the backward Fourier transform (line) of the EXAFS data between 1 and 4 Å. The real part (C), the imaginary part (D), and the magnitude (E) of the Fourier transform of the EXAFS data between 2.0 and 10.2 Å–1 are shown.

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FIGURE 3

U LIII edge EXAFS data from an aqueous uranyl carbonate species. (A) X-ray absorption data (symbol) and background function (line). (B) EXAFS data, x(k) · k, obtained as the difference between the X-ray absorption data and the background function shown in panel A, with the backward Fourier transform (line) of the EXAFS data between 1 and 4 Å. The real part (C), the imaginary part (D), and the magnitude (E) of the Fourier transform of the EXAFS data between 2.0 and 10.2 Å–1 are shown.

Schematic of the X-ray absorption process. The hashed circle represents an X-ray-absorbing atom that has emitted a photoelectron that travels as a wave away from the absorbing atom. The concentric circles (solid lines) represent the crests of the photoelectron wave as it propagates away from the absorbing atom. This photoelectron is scattered from the surrounding atoms (black filled circles), creating a scattered photoelectron, represented by the dashed concentric circles. The interference between the photoelectron wave and the scattered photoelectron waves at the absorbing atom modulate the probability of X-ray absorption. As the incident X-ray energy is increased, the wavelength becomes smaller, and the amplitude of the interference of the two waves modulates between a maximum and a minimum. The frequency of the signal is related to the distance (R0) between the absorbing atom and the neighboring atoms. All the atoms at a given radial distance contribute the same signal.

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FIGURE 4

Schematic of the X-ray absorption process. The hashed circle represents an X-ray-absorbing atom that has emitted a photoelectron that travels as a wave away from the absorbing atom. The concentric circles (solid lines) represent the crests of the photoelectron wave as it propagates away from the absorbing atom. This photoelectron is scattered from the surrounding atoms (black filled circles), creating a scattered photoelectron, represented by the dashed concentric circles. The interference between the photoelectron wave and the scattered photoelectron waves at the absorbing atom modulate the probability of X-ray absorption. As the incident X-ray energy is increased, the wavelength becomes smaller, and the amplitude of the interference of the two waves modulates between a maximum and a minimum. The frequency of the signal is related to the distance (R0) between the absorbing atom and the neighboring atoms. All the atoms at a given radial distance contribute the same signal.

(Left) Fourier transforms of EXAFS data for bulk uraninite and nanoparticulate uraninite samples having the same local structure. The decrease in amplitude is due to the small particle size of the uraninite produced by green rust. (Right) Fourier transforms of EXAFS data from U(VI) reduced by green rust, processed with three different k-weights. The data processed with k-weights of 1 and 2 have been normalized to the first-shell O signal of the k-weight 3 data. The relative increase in the signal at 3.5 Å, compared to the first-shell O signal, indicates a heavy-atom neighbor (U).

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FIGURE 5

(Left) Fourier transforms of EXAFS data for bulk uraninite and nanoparticulate uraninite samples having the same local structure. The decrease in amplitude is due to the small particle size of the uraninite produced by green rust. (Right) Fourier transforms of EXAFS data from U(VI) reduced by green rust, processed with three different k-weights. The data processed with k-weights of 1 and 2 have been normalized to the first-shell O signal of the k-weight 3 data. The relative increase in the signal at 3.5 Å, compared to the first-shell O signal, indicates a heavy-atom neighbor (U).

Results of XRF microprobe analysis of the spatial distribution of P, Ca, Ni, and Fe on an amorphous lepidocrocite thin film (~1,000 Å thick), depicting Shewanella oneidensis MR-1 on the surface. The XRF intensities are correlated to intensities shown in the scale bar.

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FIGURE 6

Results of XRF microprobe analysis of the spatial distribution of P, Ca, Ni, and Fe on an amorphous lepidocrocite thin film (~1,000 Å thick), depicting Shewanella oneidensis MR-1 on the surface. The XRF intensities are correlated to intensities shown in the scale bar.